Chain transfer agents are used to control the molecular weight of low density polyethylene. Highly active chain transfer agents (Cs>1) are desirable in a free-radical polymerization, for example, a tubular free-radical process, to produce low density polyethylene (LDPE) with a narrow molecular weight distribution. LDPE with a narrow molecular weight distribution imparts improved optics in films prepared from such polymers. However, such conventional chain transfer agents typically result in undesirable byproducts in the final polymer product. Therefore, this is a need for new free-radical polymerization processes, using new chain transfer agents, to form ethylene-based polymers, such as LDPE, with narrow molecular weight distributions. There is a further need for such processes, in which small amounts (ppm levels) of chain transfer agents act as “catalytic type” agents to minimize undesirable byproducts in the final polymer.
Huff et al., Reaction of Polymeric Radicals with Organoaluminum Compounds, Journal of Polymer Science: Part A, 1963, 1, 1553-1572, discloses the reactivities of polystyryl, polymethyl methacrylyl, and polyacrylonitryl radicals, with thirteen organometallic substrates, chosen from Groups 2b, 3a, 4a, and 5a of the Periodic Table.
Huff et al., The Reaction of Styryl Radicals with Organoaluminum Compounds, J. Am. Chem. Soc., 1960, 82, 4277-4281, discloses the reaction of styryl radicals with organoaluminum compounds.
Grotewold et al., Triethylaluminum as a Concentrate-Dependent Coinitiator and Chain-Transfer Agent of Free-Radical Polymerization of Methyl Methacrylate in the Presence of Benzoquinone, Journal of Polymer Science: Polymer Chemistry Edition, 1977, 15, 393-404, discloses the polymerization of methyl methacrylate (MMA) in the presence of triethylaluminum (TEA) and benzoquinone (BQ).
Milovskaya et al., Synthesis and Characteristics of Polystyryl Aluminum Derivatives and Their Reaction with Benzoyl Peroxide, Polymer, 1982, 23, 891-896, discloses the polymerization of styrene in the presence of triethylaluminum as a chain transfer agent.
Götz et al., Influence of Aluminum Alkyl Compounds on the High-Pressure Polymerization of Ethylene with Ternary Metallocene-Based Catalysts. Investigation of Chain Transfer to Aluminum, Macromol. Mater. Eng., 2002, 287, 16-22, discloses the influence of aluminum alkyl compounds on the metallocene-catalyzed, high pressure polymerization of ethylene.
U.S. Pat. No. 6,521,734 discloses a low-density polyethylene resin for laminates. The low-density polyethylene resin is obtained by a high pressure, radical polymerization, and has a density of 0.910 to 0.935 g/cc, a melt flow rate of 0.1 to 300 g/10 min, and a terminal vinyl group number of “0.4 or more per 1,000 carbon atoms.” This patent also discloses that a laminate having high interlayer adhesion strength can be obtained by low-temperature, high-speed molding, so that the occurrence of smoking and odor can be prevented. The laminate is suitable for food wrapping materials and containers.
U.S. Pat. No. 5,539,075 discloses an unsaturated ethylene copolymer, a method for producing this ethylene copolymer, and the use of the ethylene copolymer in compositions for producing cross-linked structures, such as materials for electric cables. Ethylene and at least one monomer are polymerized at a pressure of about 100-300 MPa, and a temperature of about 80° C.-300° C., under the action of a radical initiator. The at least one monomer is copolymerizable with ethylene, and includes a polyunsaturated comonomer having a chain of at least eight carbon atoms and at least two non-conjugated double bonds, of which at least one is terminal. The polyunsaturated comonomer preferably is an α,ω-alkadiene having 8-16 carbon atoms, most preferred 1,9-decadiene. The polymerization may also involve another vinyl-unsaturated monomer, preferably containing at least one functional group selected from hydroxyl groups, alkoxy groups, carbonyl groups, carboxyl groups and ester groups. The ethylene copolymers are disclosed as having an increased degree of unsaturation, which can be used for cross-linking the ethylene copolymer or grafting reactive groups.
U.S. Publication 2010/0108357 discloses a crosslinkable polymer composition comprising the following properties a) an unsaturated polyolefin having a total amount of “carbon-carbon double bonds per 1000 carbon atoms” of at least 0.38, and b) at least one crosslinking agent. The crosslinking agent is a carbon-carbon initiator that is free of peroxide groups, and capable of thermally decomposing into carbon-based free radicals by breaking at least one carbon-carbon single bond (for example, 3,4-dimethyl-3,4-diphenylhexane).
Additional polymerization processes and/or polymer products are disclosed in the following references: Gridnev et al., Catalytic Chain Transfer in Free-Radical Polymerizations, Chem. Rev., 2001, 101, 3611-3659; Mortimer, Chain Transfer in Ethylene Polymerization. VII. Very Reactive and Depletable Transfer Agents, Journal of Polymer Science: Part A-1, 1972, 10, 163-168; International Publication Nos. WO 2010/042390, 2011/019563, 1997/45465; European Application Nos. EP 2256158A1, EP2256159A1; U.S. application Ser. No. 12/701,859; and U.S. Pat. No. 7,767,613.
As discussed above, there remains a need for new free-radical polymerization processes, using new chain transfer agents, to form ethylene-based polymers, such as LDPE, with narrow molecular weight distributions. There is a further need to minimize undesirable byproducts in the final polymer. There is also a need for such ethylene-based polymers, such as LDPEs, that have high vinyl levels and lower levels of incorporated α-olefin chain transfer agents, as compared to, for example, conventional LDPEs with high vinyl levels. These needs and others have been met by the following invention.